1 98 H. C. COOKE 



original nature of the pebbles is difficult. About 60 to 70 per cent 

 of them are of granite; the remainder includes diorite, aplites, and 

 biotite and hornblende schists. 



Bancroft considers these sediments as probably unconformable 

 on the series of lavas that lies to the south. The structure is diffi- 

 cult to determine, as the granite intrusions have cut off the sedi- 

 ments to the north, and have stoped their way to the lavas. 

 Bancroft is of the opinion that the structure is that of a tight 

 syncline; however, it appears equally probable that the present 

 band represents only the south limb of a syncline, whose north 

 limb has been cut off by granite. The writer inclines to the belief 

 that the structure of the whole belt between Mattagami Lake and 

 the National Transcontinental Railway is a great anticlinorium, 

 the Mattagami series of sediments representing the north limb, the 

 sediments of the Pontiac series the south limb. This theory, 

 however, is unsupported by facts other than the petrographic 

 resemblance of the sediments of these two groups and a general 

 parallelism in their strike. It can probably never be proved by field 

 evidence, as outcrops over the intervening territory are too small 

 and scattered to make any continuous determination of structure. 



To the south of the Mattagami series lie the supposedly older 

 rocks, consisting of lavas, intrusives, and tuffs. The lavas include 

 andesites and basalts principally, some of them distinguished by 

 beautiful ellipsoidal structures. Slaty tuffs were observed in a few 

 places. The strike of these rocks, where observed, is in general east- 

 west, like that of the overlying sediments. The most important 

 intrusive in the vicinity of Mattagami Lake is a large body of very 

 coarse, feldspathic gabbro, petrographically identical with the 

 gabbros which in other localities have been observed to grade into 

 anorthosites, and probably to be correlated with them. It intrudes 

 the lava series, but does not ascend as high as the contact with the 

 sedimentary series; hence its relation to the sedimentary series is 

 indefinite. 



The succession in the Mattagami area therefore appears to be: 



Mica schists Basic intrusives 



Conglomerate Basalts, andesites, rhyolites, and 



Uncomformity tuffs 



